To run an MPI job you must use the command mpirun. I will explain that command later. Here you can see the example of its usage on a simple MPI program which will be our first example:
gustav@jupiter:~/mpi/simplempi 213 $ mpirun -w N `pwd`/first -- `uname -n` Greetings to the master (jupiter, 5) from (cisr, 0) Greetings to the master (jupiter, 5) from (mercury, 1) Greetings to the master (jupiter, 5) from (venus, 2) Greetings to the master (jupiter, 5) from (earth, 3) Greetings to the master (jupiter, 5) from (mars, 4) Greetings to the master (jupiter, 5) from (saturn, 6) Greetings to the master (jupiter, 5) from (uranus, 7) Greetings to the master (jupiter, 5) from (bacchus, 8) Greetings to the master (jupiter, 5) from (ceres, 9) Greetings to the master (jupiter, 5) from (diana, 10) Greetings to the master (jupiter, 5) from (minerva, 11) Greetings to the master (jupiter, 5) from (vesta, 12) gustav@jupiter:~/mpi/simplempi 214 $